Higher plant food intake is associated with lower cancer risk in humans. Bioactive constituents present in plant foods or derived from them in the human GI tract have specific effects on xenobiotic metabolism; however, evidence regarding mechanisms of cancer risk reduction in humans is almost totally lacking. To test the hypothesis that induction of Phase II enzyme systems through increased intakes of vegetables and fruits is a plausible mechanism for reduced cancer risk, we propose to examine the effects of diet on glutathione S-transferase isoenzyme activity in humans.